This invention relates to pipette devices for use with removably mounted tips and more particularly to pipette devices having mechanisms for ejecting the removably mounted tips.
Most hand held manual or electronic pipettes have a mechanism for ejecting a disposable tip secured to the shaft of the pipette. Prior to the recently developed LTS System of the Rainin Instrument Co., Inc. of Emeryville, Calif., which is characterized by pipette tip mounting and ejection forces of less than one pound, the static holding friction or mounting forces required for retaining a tip in a fluid tight sealed condition on the shaft of a pipette is typically greater than four to six pounds in order to withstand the lateral forces exerted on the tip during touching off in normal pipetting activities. During pipette tip ejection such frictional retention or mounting forces must be overcome in order to start moving the tip off of the shaft. The required peak pipette tip ejection force is typically in the range of eight to twelve pounds, but can be as high as 20 pounds. Once the tip begins to move off of the pipette shaft, the force required to continue moving the tip reduces to approximately 50% to 60% of the frictional retention force.
One common tip ejector mechanism is a spring biased rod with a thumb actuated button on its upper end. The lower end of the rod is secured to a collar positioned adjacent the upper end of the disposable tip. See, for example, U.S. Pat. No. 3,991,617 and U.S. Reissue Pat. No. 32,210. When the user presses down on the button, the lower end of the collar presses against the tip. The user must supply an ejection force which equals or exceeds the frictional retention force in order to eject the tip from the shaft of the pipette. Most users do not have sufficient time following movement of the tip down the shaft to reduce the applied ejection force from the initial ejection force. Instead, the user continues to exert the peak ejection force until the ejector impacts the bottom stop of the tip ejector mechanism. Static stress on the order or eight to twelve pounds followed by a rapid movement and a sudden stop, causing an impact on the thumb, can contribute to repetitive motion injuries to the hand and wrist when repeated many times daily over long periods of time.
Several different approaches have been used to reduce the stress in a user""s thumb or finger(s) from tip ejection forces. One such approach is to use a mechanical advantage, for example by means of cams, gears or a lever mechanism, to reduce the forces required to eject a tip. See for example U.S. Pat. Nos. 4,779,467 and 5,435,197. These reduced forces, however, come at the expense of additional motion required by the user""s thumb or finger(s). The total energy or work supplied by the user""s thumb or finger(s) is at least as much as that required for the traditional push rod mechanism. Furthermore, practical designs are limited to a mechanical advantage of 2:1, because of limitations on the accompanying travel distances and time, and are thus capable of reducing the forces only by a factor of two. Another approach is to reduce the frictional retention force holding the tip on the pipette shaft. One such solution uses an O-ring on the shaft to form a soft, compliant seal with the inside surface of the tip. See in this regard the Transferpette multichannel pipettes from BrandTech Scientific Inc. of Essex, Connecticut. Unfortunately, the lower retention force provided by such O-ring seals come at the cost of reduced sealing reliability and increased maintenance as well as increased possibilities of contamination.
Other approaches for reducing tip ejection forces focus on the tip. For example, U.S. Pat. Nos. 4,072,330 and 4,748,859 disclose a disposable tip with increased compliance for decreasing frictional retention force. These devices, however, suffer from decreased lateral tip stability.
Another approach uses a motor driven tip ejector mechanism. See for example U.S. Pat. No. 4,399,712. This approach minimizes stress on the user""s thumb or finger(s), but suffers from disadvantage that the direct drive must have sufficient strength to generate the peak force required to eject a tip without stalling or causing undue wear on the mechanism. In addition, excess stroke distance must be provided at the end of the normal pipette cycle to eject the tip. As a result, additional head space volume must be added to accommodate the extra piston stroke distance and the pipette body must be lengthened. Another motorized ejector mechanism is described in U.S. Pat. No. 4,616,514 and utilizes a proprietary tip design having a soft seal on the end of the tip for improved sealing and easy tip ejection.
As can be seen from the foregoing, many of the current solutions for minimizing the stress on the hand and/or wrist of a pipette user from tip ejection have accompanying disadvantages. It would be desirable to develop a new pipette which overcomes these disadvantages.
In general, the invention provides a pipette for repeatedly aspirating and dispensing a predetermined quantity of liquid. The pipette includes a hollow body having first and second extremities. The second extremity is adapted to removably receive the pipette tip. An ejector is carried by the body and has a first extremity disposed within the first extremity of the housing and a second extremity movable vertically about the second extremity of the housing. The ejector is movable from a first position for permitting the pipette tip to be securely mounted on the second extremity of the housing and a second position for pushing the pipette tip off of the second extremity of the housing. A spring and means for compressing the spring so as to store energy in the spring are carried by the housing. Locking means is additionally carried by the housing for retaining the spring in the compressed position. Release means is provided for releasing the locking means so that the ejector is driven by the spring to the second position to move the pipette tip distally on the second extremity of the housing.